According to the instant description, the expression “bituminous product” intends to depict a bitumen or a composition including a bitumen together with additional ingredients such as aggregates and/or additives, such as an asphalt for example.
“Polymer modified bitumen” refers to a bitumen containing an additional polymer, for example an elastomer such as rubber or a thermoplastic elastomer, said polymer having preferably a good enough dispersibility and/or solubility in bitumen.
Bituminous products are commonly known and may be used in many fields, e.g. mixed with mineral aggregates, to produce paving materials or coatings, especially on roadways. There are also used for imparting waterproofing, for example for the production of roofing felt and for sealing flat roofs.
As regards their application for producing paving materials on roadways, bituminous products are commonly used as a bituminous binder together of mineral fillers, such as aggregates, granulates, gravels, sand and fines. The association of the bituminous binder and the mineral filler is commonly referred as being a so-called “bituminous coated material”, “asphalt concrete” or “bituminous mix” (“enrobé bitumineux”, in the French language).
It is well known that bituminous coated materials consisting of a consolidated mixture of bitumen and aggregate are not satisfying as such, and that they exhibit many drawbacks which render them improper to a direct use, i.a. in the field of roadways. Especially, a raw bituminous coated material is highly temperature-sensitive, with a relatively poor adherence between the mineral filler and the bituminous binder, and has a poor resistance to strain and mechanical shocks.
In roadways application, especially in the upper layer of the road covering which is in contact with the tires of the vehicles, the bituminous coated material generally have a content of bituminous binder of about 2 to 10%, typically of about 5% by weight. With such high concentrations of bituminous binder, the role of the binder is of major importance, which may lead to some troubles if the bituminous coated material is used as such. Especially, it may lead to thermal fractures, fatigue and/or rutting.
More precisely, at low temperatures (typically −10° C. or less), the bituminous binder is in a vitreous state and is therefore brittle. This brittleness, associated to the heterogeneity of the bituminous coated material, often lead to microcracking which form unsatisfying transversal cracks, due to the thermal stress.
At higher low temperature (especially from −5 to 0° C.), some other cracks may still appear, which generally lead to longitudinal cracks, generally interconnected.
On the other hand, at high temperatures (typically above 50° C., especially of about 60° C.), the bituminous binder becomes visco-plastic, and more and more fluid when the temperature increases. Therefore, intensive passages on the roadways of heavy masses such as vehicule axles including trucks induce a permanent deformation of the bitumen and therefore of the roadway track. This phenomenon is the source of the rutting damage.
The formation of such cracks, and/or rutting have to be avoided, especially as the bituminous binder furthermore ensures a waterproofing of the roadway, which protects the basis of the roadway.
Hence, especially for roadways application, it is sought for a bituminous binder that it preferably exhibits the following properties:                a good resistance to cracking at low temperature, especially at −10° C. or below;        a low deformation at high temperature, especially at 50° C. or above;        a high resistance to fatigue, so as to provide long term durability.        
To this end, additives have been proposed for improving the properties of the bituminous products and overcome the deficiencies of bituminous binders.
In this connection, it is e.g. known to make use of additives which are elastomeric polymers. However, such elastomeric polymers have some drawbacks.
Especially, elastomeric polymers are not easily dispersible in the bitumen, and precautions have to be taken in order to obtain a suitable homogenous dispersion of such additives in a bituminous product.
Moreover, phase separation phenomenon between the elastomeric polymer and the melt bitumen tends to occur, especially during storage. Therefore, elastomeric polymers containing bituminous products may dramatically change upon storage. So as to avoid such phase separation, there is a need for complex installations which can maintain high temperatures and continuous mixing before the use of the modified bituminous product.
For these reasons, it is commonly made use of sulphur in bitumen products modified with elastomeric polymers, since it allows avoiding the above drawbacks. It is advanced that sulphur favours interactions (bounds) betweens the polymer and the bitumen, leads to a reinforcement of such interactions and also favours and reinforces a crosslinking upon heat treatment.
Many forms of sulphur may be uses, including elementary sulphur and sulphur donors. Typically, sulphur is incorporated in the form of powder, granulates and/or scales, optionally with other additives.
The benefits of the sulphur introduction are however by some drawbacks, which rise in practice when incorporating the sulphur in the bitumen product. Especially, the use of sulphur or sulphur containing products leads to a production of toxic gaseous product, such as hydrogen sulphide. Then, again, there remain a need of complex installations and/or a need of carrying out the incorporation of the sulphur at low temperature, which extends the duration of the processes.
Processes which implement sulphur and/or sulphur compounds are e.g. disclosed in U.S. Pat. Nos. 6,767,939 and 7,402,619, wherein the described methods systematically lead to a side production of hydrogen sulphide.
Alternatively, it has been proposed to make use of sulphur-free dispersing agents in bitumen modified by polymer, e.g. 2-ethylhexyl acid phosphate as contemplated in US 2004/0249024. Such a dispersing agent reveals however difficult to be carried out and is not compatible with common installations for bitumen treatment.